The birth of twin calves in cattle is uncommon, occurring in 0.5% of beef cattle pregnancies and up to 5% in high-producing dairy breeds. When twins are born, a specific biological concern arises regarding the fertility of the female offspring, particularly when the twins are of different sexes. The physiological processes during gestation can lead to a form of sterility unique to this species, raising the question of whether twin-born cows can reproduce.
Defining the Freemartin Phenomenon
The answer to sterility lies in freemartinism, a condition that specifically affects a female calf born co-twin to a male calf. A freemartin is genetically female but exhibits male characteristics, resulting in infertility. This is a consistent outcome of mixed-sex twin pregnancies and one of the most common forms of sexual abnormality observed in cattle.
The female twin is rendered sterile in 90% to 95% of cases due to the failure of her internal reproductive organs to develop properly. Conversely, the male twin is usually fertile, though a small percentage may experience reduced fertility. Identifying a freemartin calf early is important for herd management, as the animal cannot contribute to reproduction.
The Biological Mechanism of Sterility
The biological cause of freemartinism is the shared blood supply between the two fetuses during a mixed-sex pregnancy. The bovine placenta allows the blood vessels of the separate fetal membranes to fuse, a process called placental vascular anastomosis, beginning early in gestation (around day 30 to 40). This fusion creates a common circulatory system, exchanging blood and cells between the male and female fetuses.
This exchange results in chimerism, where the female calf has a mixture of her own female cells (XX) and the male’s cells (XY) circulating in her system. The male fetus’s testes develop earlier than the female’s ovaries, producing masculinizing hormones like Anti-Müllerian Hormone (AMH) and testosterone. These hormones cross the shared blood barrier and enter the developing female fetus.
The male hormones disrupt the normal sexual differentiation process. AMH causes the regression of the Müllerian ducts, the embryonic structures that normally develop into the fallopian tubes, uterus, and part of the vagina. This hormonal interference leads to a severely underdeveloped or malformed reproductive tract, including small, non-functional ovaries and the absence of a uterus, resulting in permanent sterility.
Distinguishing Affected Twins
Determining if a female calf born co-twin to a male is a freemartin requires both visual inspection and specific diagnostic tests. If twins are the same sex, such as two females or two males, they are almost never affected by freemartinism because the hormonal imbalance does not occur. The concern is isolated to the female of a mixed-sex pair.
Visual and Physical Examination
Visual signs can offer early clues, with some freemartin heifers displaying a prominent, enlarged clitoris or an unusually short distance from the vaginal opening to the cervix. A more reliable field test involves measuring the depth of the vagina shortly after birth using a lubricated probe or a specialized test tube. In a young, normal heifer calf, the probe can typically be inserted five to seven inches, while a freemartin’s underdeveloped vagina often restricts penetration to three inches or less.
Definitive Diagnosis
For a definitive diagnosis, a blood test checks for chimerism—the presence of male (XY) cells in the female’s bloodstream. Finding these cells confirms that the placental fusion occurred and the calf is a freemartin, even if the external genital appearance is near-normal. In very rare instances, a female calf may be born as a single calf but still be a freemartin if her male twin died early in gestation, but only after the vascular connection and hormonal transfer had already taken place.